XR4 Overheating

[JFish]

I have an '89 Mercury 150 XR4. For some time now, I've been having a problem with overheating, even after replacing impellor and associated parts, poppet valve, and thermostats. When on plane traveling slow, the water pressure is 10 psig and the engine will overheat rapidly. When I increase throttle, the water pressure will fall off 4-5 psig (poppet valve opening)and the temperature will quit rising but not cool down. A full throttle, 20-24 psig of water pressure is achieved but still running at or near the hi-temp alarm (approx. 7/8 scale of the C-H temperature gauge). This is very frustrating because I cannot run on plane at a slow pace. Could the temperature sending unit be bad? Isn't the temperature sending unit and the temperature gauge independent of each other?

 

[ghost69_98]

Re: XR4 Overheating

I am having the same problems. The sending unit is independent of the gauge. I was told to check the plugs and do a flush. It may be picking up sand and crystalizing.

I have an '89 Mercury 150 XR4. For some time now, I've been having a problem with overheating, even after replacing impellor and associated parts, poppet valve, and thermostats. When on plane traveling slow, the water pressure is 10 psig and the engine will overheat rapidly. When I increase throttle, the water pressure will fall off 4-5 psig (poppet valve opening)and the temperature will quit rising but not cool down. A full throttle, 20-24 psig of water pressure is achieved but still running at or near the hi-temp alarm (approx. 7/8 scale of the C-H temperature gauge). This is very frustrating because I cannot run on plane at a slow pace. Could the temperature sending unit be bad? Isn't the temperature sending unit and the temperature gauge independent of each other?

 

[Chris1956]

Re: XR4 Overheating

Jfish, First of all 24PSI is very high pressure for a Merc. With this amount of pressure, that motor should not overheat. I would take off the cowling and put your hand on the heads and run it for a while. See if it really gets hot. If you do not burn your hand touching it, it is running cool enough. The thermostats open at about 140 degrees F. You can touch this temp for 10 sec or so easily.

 

[hkeiner]

Re: XR4 Overheating

The below troubleshooting guide is for 1992 through 2002 Merc OBs. However, the "possible causes" listed in this guide may still be relevent to you.

 

[j_martin]

Re: XR4 Overheating

I ran over a rotten log and cut it in half with my XR4. It did that (high pressure and overheat) until I took off the exhaust chest cover and cleaned out all the toothpicks stuck in water passages.

That's the trouble, it's plugged up somewhere, and you might have to pull the powerhead to get at it if it's in the exhaust plate. Looking for the plug up, exhaust chest is easiest, then heads, then the exhaust plate.

hope it helps
John

 

[JFish]

Re: XR4 Overheating

Thanks for the input. I guess my next step is to look for restriction in the water outlet. One question, what are you calling the exhaust chest?

JFish

 

[j_martin]

Re: XR4 Overheating

All the exhaust ports dump into a box (chest) between the cylinders. There's a cover, gasket, divider plate (keeps cylinders from blowing exhaust into the opposite cylinder), and another gasket in a stack with a gazillion bolts in it on the back of the engine. It's underneath the coils,etc. If you have a single, water cooled 40 amp electrical system, it is part of that assembly.

Like most modern engines, not hard to work on once all the junk is out of the way.

hope it helps.
John